Omitting the Relative Pronoun in Restrictive Clauses

In the following examples, the man is the subject of both verbs and cannot be omitted:

The man who told me is coming later.

The man that told me is coming later.

In the following examples, the woman is the object of the verb 'saw' and, therefore the pronoun can be omitted:

The woman who I saw is coming later.

The woman whom I saw is coming later.

The woman that I saw is coming later.

The woman I saw is coming later.

NB - WHOM is an object pronoun. It is used in formal English after a preposition and can be used to replace an object,although many no longer do this. In a phrase like 'To whom it may concern', who would not be acceptable.